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What is ACARS ?


ACARS is a system of communications between aircraft and their ground stations. Using an air band radio and a commercially available decoding device, you can read the messages that are passed between the aircraft and the company operations offices. For more detailed look at ACARS, visit this page.


What type of radio do I need ?


Typically any VHF radio capable of receiving the aircraft band which ranges from 118.0 mhz to 136.975 mhz. Direct tuning capability is preferred. Several radios have provided excellent results: Bearcat BC350A, BC9000XLT; Radio Shack Pro-2006. Results will vary upon location, proximity to sources of interference etc.


What type of antenna do I need ?


Try to find an antenna that is tuned (or is tunable) to the aircraft frequencies. Typically, one that can be located as high as possible and as far away as practical from the computer provides the best results. Follow all safety precautions when installing and using any external antenna.


What type of PC is needed ?


Enthusiasts have successfully monitored ACARS using a 386DX 33 carrying 4Mb RAM and fitted with a low capacity hard disk through to the latest Pentium III models. The latest version of the Lowe 2000 however performs much better with a Pentium processor and at least 16Mb of memory. A newer version SoundBlaster card is required to run Kracars and WACARS freeware decoders.


Is there a decoder for the Macintosh ?


The only thing that we have been advised of is a freeware program for the Mac. For more details, go to: http://www.blackcatsystems.com/software/multimode.html


What else will I need ?


You will need a decoder that will interface your PC to your radio. In some cases the decoder takes the form of a demodulator plug that connects to your PC and the radio, or in another example, the SoundBlaster card can be used as the interface. Also available are standalone units and internal PC cards.


The most popular ACARS interface is the Lowe Airmaster 2000. It uses a demodulator plug and a software system to decode signals sent out of the external speaker jack of the radio. The maker of this product is Lowe Electronics in the U.K.


Universal Radio makes several decoding units that are either stand alone or are made as cards that can fit into an open PC slot on the motherboard. Universal is located in Reynoldsburg Oh. Look at the online Universal catalog.


Two shareware products are also available, Kracars and WACARS. Both products are a very affordable method of getting up and running fairly inexpensively. Both require SoundBlaster (or compatible) cards to demodulate signals fed from the audio output of the radio.


Another entry into the decoder market is SkySpy. For more information check them out on-line by clicking here.


I don't live an airport, does that matter ?


Not really, as you will probably decode a number of transmissions from aircraft flying overhead (see the advanced section to figure out your effective range). If you are in close proximity to a busy airport you will hear transmissions from the ground to the aircraft as well as the easier to hear transmissions from the aircraft to the ground station. If you live away from an airport, you will probably only hear transmissions from the aircraft to the ground.


Is ACARS available worldwide?


Unfortunately not. ACARS is most prevalent in Europe, North America, the Pacific Rim, and scattered parts of Africa, Southl America and the Middle East.


Are there any books on ACARS?


The most authoritative book on ACARS is "understanding ACARS" by Ed Flynn. This publication will be useful for beginners and experts and is loaded with valuable information about decoding ACARS transmissions. The book can be ordered from Amazon.com.


Is monitoring ACARS legal?


It is legal in the United States under certain conditions. In other countries the monitoring and or divulgence of aviation transmissions is prohibited by law. Contact your local radio monitoring society for more information.




What does an ACARS transmission sound like?


Tune your radio to one of the ACARS frequencies and you should hear sounds like you do on this sound byte: ACARS.wav(227kb) Note the difference in the signal strength of the different bursts. Clearly you have a better chance at decoding the louder signals than the softer ones. Also note that there is a lot of hiss, but the signal is virtually free of interference.


Are ACARS Signals AF, FM or WFM?


All ACARS transmissions are AM. If your bursts don't sound like the wav file above, then check to make sure you are receiving in the AM mode (Some receivers have selector switches, others are automatic).


Why don't I get decodes from certain airlines?


Some airlines do not use ACARS. Examples are Southwest Airlines, Frontier, & Midway. It is also possible that the airlines you are looking for, don't fly within your reception area.


Why don't I get decodes from certain aircraft types of an airline?


Some airlines that use the ACARS system, do not have all of their aircraft equipped with ACARS. For example, all of Delta's fleet uses ACARS except their B727's. In the case of Air India, a handful of their B747-400's, but none of their B747-200's use ACARS.


I get decodes from airlines that are no longer in business, why?


A number of US Airways aircraft use their old PI airline codes, along with their previous registration markings. Similarly, there are a number of ex Western Airlines aircraft now flying for Delta that still use the WA airline code. Go to the ACARS-Link anomalies page for more details.


Who uses flight number GS0001?


This is a code assigned to general aviation aircraft, mainly executive jets. In addition, some Atlantic Coast Airlines flights were using this flight number. Another variation on this is UV0001 which are general aviation aircraft under the dispatch of Universal Radio ( an aircraft dispatch company in Houston Texas).


Is a message an uplink or downlink?


This is not always easy to figure out. Usually the message content is a dead giveaway if there is free text in the message. Departure clearances for example are uplinks to the aircraft deetailing the flight plan the aircraft is to follow. The request for "one cart and two wheelchairs" is a downlink from the aircraft to the ground station requesting equipment to meet the aircraft.


In the Lowe Airmaster 2000 software, a general rule of thumb to be used is that message labels that are pure alpha characters are uplinks, and numeric or alphanumeric values represent downlinks. Note: recent uplinks to biz-jets (GS0001) have been in numeric format, so this is not always true. One of the features of the Lowe Airmaster 2000 is that there is a user maintained file where uplink message labels can be identified, and the software displays the word "uplink message" when one of these message labels is decoded.


Why are most messages received, downlinks?


This is probably because you are located too far away from the transmitter where the uplinks are sent from. The reception of signals transmitted from aircraft at 35,000 ft is much greater than the reception from ground based stations.

One way to receive uplinks is to take your gear out to the airport and set up (assuming your equipment is portable). You will receive a lot more uplinks, and in general you will see a lot more detailed text messages. Caution: make sure your local airport authorities allow you to set up !


Can aircraft send ACARS messages to other aircraft?


As far as we know, messages cannot be sent directly from one aircraft to another. On a number of occasions, messages have been received from aircraft with codified text in the free text section indicating that the message be routed to another aircraft. It therefore appears that messages can be sent from one aircraft to another, but only via a ground station. Air Canada have been noted using this technique, along with some business aircraft.


Why do I get so many messages with the message label Q0?


A Q0 message is a sort of radio check, so the aircraft frequently send out these message to ensure that communications are established correctly between the aircraft and ground stations.


How do I minimize interference from my PC?


PC's and radios generally don't get along well. The trick is to keep the PC as far away from the radio as practical. Another tip is to keep any cables running between the PC and the radio away from other cables plugged into the PC (ie: monitor cables, keyboard, modem/phone cables etc.). Also, try turning the monitor off and see what effect that may have. Finally, a good quality cable for the antenna and between the PC and radio will get better results.


Different equipment produces different levels of interference. Unfortunately, the best remedy make take some experimentation, and a lot of patience.


Can you scan ACARS frequencies successfully?


In some cases, the answer is yes, from Bernard Eccleston:


"My set up is a Win 108, Lowe Airmaster 3.0 and an air band antenna mounted externally, continually producing 2mb plus log files per day. The criteria is, a scanner with a sensitive squelch, a bit of patience, and a willingness to experiment. Locking onto one frequency is the main problem, after extending the Airmaster lead, by slotting in a couple of audio extension leads (mono or stereo) to get the set away from the source the problem disappeared.


Enter the ACARS frequencies into your scanner alternately, using 10 memory slots i.e. 131.525 & 131.725 . After various experimenting with the squelch settings the optimum was little or none at all, i.e. a quiet radio.


Experiments were carried out using these Scanners:


* Win 108 - Excellent results no loss of quantity or quality

* Yupiteru MVT-7100 - Excellent results no loss of quantity or quality

* Realistic Pro-43 - High loss of quality and quantity

* Uniden Bearcat 100XL - High loss of quality and quantity


All these scanners were used with an externally mounted air band antenna. Best results will always be achieved with an external aerial as internal ones can pick up more interference. "


The power went out on my PC..Can I retrieve my logs?


If you have a Scandisk type program or Norton disk doctor, the answer is yes !


When the power was interrupted to the system, the file that decoder was logging to became corrupt. When the above programs are run, they will give a message saying that lost clusters have been found on the disk. Click on the button to repair the problem, and take the option to save the data as a file. It should then create one or more files, usually named FIL000... in the root directory. In some PC configurations, you may get Scandisk or Norton when you boot back up after the power failure, be careful not to delete the corrupted files by just blowing through the prompts on the boot up.


Use a text editor (or MS Word, Word Perfect etc.) to find which of the FIL000 files contains the logged data. By viewing the file in an editor you will see that all but the very end of the file is still in tact. Delete the abnormal looking data at the end of the file. Be sure not to leave any spaces or data after the last message, as programs like DACARS may not work.


Use the Editor's "save as" (or equivalent) option to save the file as a text only file. Use the naming convention that the decoding software uses (Airmaster uses DDMMHHMM.log) and save into the appropriate log folder. When you are satisfied that the restore is ok, remove the FIL000 (or equivalent) file from your root directory.


What is Airnav?


Airnav is a software package that reads ACARS messages and displays the information onto a graphical map background, showing information about an aircraft such as speed, altitude, and position.


What range can you expect for ACARS reception?

The following explanation from Dennis Rodgers provides the answer:

VHF propagation is often referred to as being "line of sight". With visual line of sight the distance is limited by the curvature of the earth. The effective radio horizon distance is slightly greater than the visual horizon distance because the wave path tends to be bent slightly by the atmosphere.

By assuming an effective earth radius which is 1.33 times its actual value the radio horizon is given by:


D = 1.42 times (the square root of H)

Where D is the horizon distance in miles and H is the height of the antenna above sea level in feet.



Height (ft, ASL) of Transmit Antenna Miles

500 32

1000 45

5000 100

10000 142

15000 174

20000 201

30000 246

40000 284

50000 318

60000 346


You may "expect" to receive transmissions at sea level from an aircraft at 20000 feet 201 miles away. Similarly if you are living (say) at 5000 feet asl and an aircraft was flying at 10000 feet asl you can approximate by adding together your height and the aircraft's height to give an effective height of 15000 feet i.e 174 miles.


That is the theory, but unfortunately in practice it is far more complex. Buildings, hills, mountains and other obstructions anywhere along the path can invalidate to differing extents the distance you will achieve in practice. In simple terms all obstructions tend to reflect/deflect/attenuate vhf signals.


For this reason try to use an outdoor antenna at the highest safe and practicable height, and clear of obstructions. Not easy in an urban location. Always ensure that your antenna is properly and SAFELY supported, and include very generous allowances for the wind forces in all conditions. Winds of 100 mph can place a very considerable loading on an antenna and its supports, but even a wind of much lower average speed with random gusts of high speed can be equally severe. Do not take chances with safety.


Complications also arise from what are known as anomalous propagation conditions caused by the atmosphere. Propagation tends to improve as the barometric pressure rises, and the converse as it falls. Other factors such as temperature inversions can give rise to 'ducting' which simulates a pipe within which VHF signals behave quite differently and travel great distances irrespective of obstructions normally in the path. If occasionally you hear an ACARS ground station much further away than you expect. You may hear it for a hours, even a few days - then zap - it is not there anymore. Do not blame your antenna or other equipment it is "conditions". It can be weeks or months before that occurs again.

This is merely intended to illustrate some of the simple considerations of VHF reception. The whole subject is vastly more complex and frustrating. Results from using the formula are no guarantee that the distances will be achieved over any specific path, or time, but must be regarded only as a very broad approximation.


All data on this Wiki copyright Mark Avey unless otherwise stated.

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